A radiation-emitting thin-film semiconductor chip is distinguished preferably by one or a plurality, particularly preferably all, of the following characteristic features:                a reflective layer is applied or formed at a first main face of a radiation-generating epitaxial layer sequence that faces toward a carrier element, said reflective layer reflecting at least a part of the electromagnetic radiation generated in the epitaxial layer sequence back into the latter;        the epitaxial layer sequence has a thickness in the region of 20 μm or less, in particular in the region of 10 μm;        the epitaxial layer sequence contains at least one semiconductor layer with at least one face having an intermixing structure which ideally leads to an approximately ergodic distribution of the light in the epitaxial layer sequence, that is to say it has an as far as possible ergodically stochastic scattering behavior;        the epitaxial layer sequence is firstly grown onto a growth substrate, the epitaxial layer sequence subsequently being stripped from the growth substrate and fixed on the carrier element.        
A basic principle of a thin-film light-emitting diode chip is described for example in I. Schnitzer et al., Appl. Phys. Lett. 63 (16), Oct. 18, 1993, 2174-2176, the disclosure content of which is in this respect hereby incorporated by reference.
A thin-film light-emitting diode chip is to a good approximation a Lambert surface radiator and is therefore particularly well suited to the application in a headlight.
Conventional radiation-emitting semiconductor chips often have a rectangular geometry for reasons of production technology. The semiconductor chips generally comprise a multilayer structure with an active, radiation-generating layer, said multilayer structure being deposited epitaxially on a carrier substrate. The carrier substrate is preferably electrically conductive in order to enable a vertical current flow. Moreover, it is expedient in many cases if the carrier substrate is transparent to the radiation generated in the active layer of the multi-layer structure. However, a high transparency is often at odds with a high electrical conductivity of the material for the carrier substrate. Thus, by way of example, sapphire used for GaN-based light-emitting diodes is transparent to blue light but is not electrically conductive. By contrast, although silicon carbide as carrier substrate for GaN light-emitting diodes is conductive and transparent, the transparency decreases as the conductivity increases, with the result that the properties of the semiconductor chip are not ideal in this case either.
GaN-based semiconductor chips generally serve predominantly for generating radiation in the blue-green spectral range and have a plurality of layers comprising a GaN-based material. In the context of this invention, GaN-based material is understood to mean not only GaN itself but also materials derived from GaN or related to GaN and also ternary or quaternary mixed crystals based thereon. In other words, “GaN-based” means in this connection that a component or part of a component designated in this way preferably contains AlnGamInl-n-mN, where 0≦n≦1, 0≦m≦1 and n+≦1. In this case, this material need not necessarily have a mathematically exact composition according to the above formula. Rather, it may have one or more dopants and also additional constituents which essentially do not change the physical properties of the material. For the sake of simplicity, however, the above formula only comprises the essential constituents of the crystal lattice (Al, Ga, In, N), even though these may be replaced in part by small quantities of further substances. In particular, these materials include GaN, AlN, InN, All-xGaxN, Inl-xGaxN, Inl-xAlxN and All-x-yInxGayN where 0<x<1, 0<y<1 and x+y≦1.
Therefore, one possibility for reducing the absorption losses and thus for increasing the external efficiency is the removal of the carrier substrate in conjunction with suitable mirror layers (thin-film concept). However, a semiconductor thin film is essentially a plane-parallel plate whose coupling-out efficiency is not increased compared with a standard diode on account of the geometry. Particularly if a carrier substrate exhibiting only little absorption (for example GaN on SiC) has already been used for the semiconductor chip, the increase in the external efficiency of the thin-film semiconductor chip is too small to justify the increased technical outlay for removing the carrier substrate.
In order to elucidate the problem area of coupling out radiation, FIG. 8 schematically shows a semiconductor chip with the cones of coupling out radiation. Radiation can be coupled out of the semiconductor chip only from a cone with an aperture angle of θ=sin−1 (next/nint), where nint denotes the refractive index of the semiconductor material and next denotes the refractive index of the surroundings. For a GaN semiconductor (nint=2.5), the coupling-out angle θ is 23° with respect to air (next=1) and 37° with respect to a plastic encapsulation (next=1.5). Radiation that is generated in the semiconductor chip and does not impinge on the interfaces within a cone is finally reabsorbed and converted into heat. Although the coupling-out cone is large for GaN systems in comparison with GaAs systems (nint=3.5), it nevertheless leads to undesirably high radiation losses.
These conditions also do not change significantly with altered layer thicknesses. However, the thin-film geometry is expedient for the beam coupled out via the top side since the absorption is low on account of the short path in the semiconductor. For the beam coupled out laterally, by contrast, the efficiency may even be lower on account of the multiple reflections in the semiconductor.
Therefore, there are already various approaches for increasing the external efficiency of semiconductor chips through altered geometries. Mention shall be made here, in particular, of a so-called micropatterning of the entire multilayer structure, which leads to an intensified lateral coupling out of radiation on account of the larger total area of the side faces of the multilayer structure. In addition, the side faces of the individual multilayer structures thus produced may be beveled. Examples of such semiconductor chips are disclosed in DE-A-198 07 758, EP-A-0 905 797 or JP-A-08-288543.
A further possibility for increasing the coupling out of radiation is shown in FIGS. 3 and 5 of DE-A-199 11 717. Here, the multilayer structure with the active, radiation-generating layer is assigned individual radiation coupling-out elements in the form of sphere segments or truncated cones formed for example by means of corresponding etching of grown layers.